1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be used to measure the concentration of a specified gas component such as a nitrogen oxide contained in exhaust gasses of an automotive engine, and more particularly to an improved structure of such a gas sensor element designed to minimize an error of a sensor output.
2. Background Art
NOx sensors which are installed in an exhaust system of an automotive engine and equipped with electrochemical cells called a monitor cell, a pump cell, and a sensor cell formed on solid electrolyte plates are typically used for burning control of the engine. The pump cell works to keep the concentration of oxygen (O2) within a measurement gas chamber at approximately zero. The monitor cell outputs an electromotive force or limiting current as a function of the concentration of oxygen within the measurement gas chamber. The output of the monitor cell is fedback to the pump cell to pump the oxygen molecules inside or outside the measurement gas chamber. The sensor cell works to dissociate NOx molecules within the measurement gas chamber and discharge oxygen produced when the NOx molecules dissociated to produce a current as a function of the concentration of NOx molecules.
FIG. 7 shows a comparative example of a NOx sensor.
The illustrated structure has the monitor cell 3 disposed in the vicinity of the sensor cell 4, which may cause oxygen ion currents to flow, as indicated by an arrow a, from the measurement gas electrode 31 to the reference electrode 32 of the monitor cell 3 and, as indicated by an arrow d, from a measurement gas electrode 41 of the sensor cell 4 to the reference electrode 32 of the monitor cell 3.
The NOx sensor has a monitor circuit 93 equipped with a power supply 932 and a monitor cell current detector 931. The monitor current detector 931 measures an undesirable current arising from the oxygen ion currents a and d. Correctly, a current flow produced in the monitor cell 3 as a function of the concentration of oxygen is provided by the sum of a and c. A difficulty is, thus, encountered in the monitor cell 3 to measure the concentration of oxygen accurately.
Similarly, a current flow produced in the sensor cell 4 as a function of the concentration of NOx is provided by the sum of b and d, but however, a sensor circuit 94 equipped with a power supply 942 and a sensor cell current detector 931 measures an undesirable current arising from the sum of b and c. An error is, thus, added to an output of the sensor cell 4.
The above problem is encountered commonly in gas sensors of the type wherein the monitor cell and the sensor cell are both employed.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas sensor which is designed to minimize an error of a senor output.
According to one aspect of the invention, there is provided a gas sensor which may be installed in an exhaust pipe of an internal combustion engine to measure the concentration of an oxygen-containing gas component which is dissociative to produce oxygen ions or an oxygen-active gas component which is reactive to oxygen ions such as NOx, CO, or HC contained in exhaust gasses for use in engine burning control and/or catalytic systems.
The gas sensor comprises: (a) a gas cavity into which gasses consisting essentially of oxygen molecules and a specified gas such as NOx, CO, or HC are admitted through a given dispersion resistance; (b) a pump cell including an oxygen ion conductive solid electrolyte member, a first pump cell electrode, and a second pump cell electrode which is exposed to the gas cavity, the pump cell being responsive to application of a voltage across the first and second pump cell electrodes to selectively pump oxygen molecules into and out of the gas cavity for adjusting a concentration of the oxygen molecules within the gas cavity to a desired value; (c) a sensor cell including an oxygen ion conductive solid electrolyte member, a first sensor cell electrode, and a second sensor cell electrode which is exposed to the gas cavity, the sensor cell being supplied with electric power through the first and second sensor cell electrodes to produce an electric current as a function of a concentration of the specified gas within the gas cavity; (d) a monitor cell including an oxygen ion-conducting member, a first monitor cell electrode, and a second monitor cell electrode which is exposed to the gas cavity, the oxygen monitor cell being supplied with electric power through the first and second monitor cell electrodes to produce an electric current as a function of a concentration of the oxygen molecules within the gas cavity; (e) a sensor cell power supply working to supply the electric power to the sensor cell; (f) a monitor cell power supply working to supply the electric power to the monitor cell; (g) a sensor cell current detector measuring the current produced by the sensor cell; and (h) a monitor cell current detector measuring the current produced by the monitor cell. The sensor cell current detector is disposed between the second sensor cell electrode exposed to the gas cavity and the sensor cell power supply. Similarly, the monitor cell current detector is disposed between the second monitor cell electrode exposed to the gas cavity and the monitor cell power supply. This enables the sensor cell current and the monitor cell current to be measured without addition of electric noises arising from current components flowing from the monitor cell to the sensor cell and vice versa.
In the preferred mode of the invention, the gas sensor further comprises a control circuit which works to control the voltage applied to the pump cell so as to bring the current measured by the monitor cell current detector into agreement with a constant value.
The gas sensor may also include a concentration measuring circuit working to determine a difference between the currents measured by the monitor cell current detector and the sensor cell current detector to provide a signal indicative of the concentration of the specified gas within the gas cavity.
The oxygen ion conductive solid electrolyte members of the sensor cell and the monitor cell may be formed by a one-piece member.